NPN transistors have two N type regions surrounding a P type region. One of the N type regions is the collector, the P type region is the base, and the second one of the N type regions is the emitter. Depending upon how the NPN transistor is voltage biased, the NPN transistor will have a different mode of operation. When the emitter-base junction is reverse biased and the collector-base junction is reverse biased, the NPN transistor operates in a cutoff mode. When the emitter-base junction is forward biased and the collector-base junction is reverse biased, the NPN transistor operates in an active mode, and when the emitter-base junction is forward biased and the collector-base junction is forward biased, the NPN transistor operates in the saturation mode. The active mode is used when the transistor is to be used as an amplifier, and the cutoff mode and the saturation mode are used when the transistor is to be used as a switch.
One parameter of an NPN transistor is the common-emitter current gain, which is often known as β or HFE. When in the active mode, the common-emitter current gain is the ratio of the collector current over the base current. For an NPN transistor, the common-emitter current gain may be in the range of 50 to 200. For a super-beta NPN (SBNPN) transistor, the common-emitter current gain may be, for example, 1,000 or more.